The rapid expansion of the number of cellular radio telephones coupled with the desire to provide additional services has prompted the development of a digital standard. The standard suggests an increase in system capacity over the previous analog system through the use of digital modulation and speech coding techniques. The standard for the cellular system is described in detail in Electronic Industries Association, Project Number 2398, January 1991, IS-54 (Revision A), entitled Dual-Mode Mobile Station--Base Station Compatibility Standard. The standard describes in .sctn.1.2 a time division multiple access (TDMA) channel 40 milliseconds long divided into six equally sized data packets 6.66 milliseconds long. A data packet is a burst of information characterized by sequentially encoded consecutive pairs of bits, commonly known as symbols.
The standard describes in .sctn.2.1.3.3.1 a linear modulation technique known as .pi./4 shifted, differentially encoded quadrature phase shift keying (.pi./4 DQPSK). The symbols are transmitted into one of four phase angles (.+-..pi./4, .+-.3.pi./4) using differential quadrature component signals producing an eight point phase constellation. The symbols are represented by a normalized magnitude vector and a phase angle. The symbols are transmitted as changes in phase rather than absolute phases.
Signal propagation in the radio frequency band, such as the 800 MHz band for cellular radiotelephones, is generally characterized by two types of channel-induced distortion: time dispersion distortion and multipath distortion. These types of distortion are caused by a rapid rate of change of the received data packet's amplitude over time and are predominantly affected by the frequency of the signal, how rapidly the receiver is moving through its environment and large objects in the vicinity of the receiver. When the amplitude over a portion of the data packet approaches a null, the symbols can be corrupted by noise present in the channel that alters the state of the symbol causing the receiver to detect wrong information.
Time dispersion distortion is usually found in an environment where a large reflecting source, such as a mountain or a tall building, is present. A receiver operating in this environment receives the data packet from a fixed source transmitter and a delayed data packet from the reflecting source. The time delay between the reception of the two data packets results in time dispersion distortion.
Multipath distortion is characterized by many components of the same data packet having different energy levels reaching the receiver at the same time. As a result, the amplitude and phase of a data packet varies over time. This variance is referred to as "Rayleigh fading" of the data packet.
The present challenge is to recover the received symbols in the data packet that were transmitted in the presence of the channel-induced distortion. Typically, the receiver is synchronized to the data packet using a process called correlation. Synchronization is described in Chapter 8 of Digital Communications, Fundamentals and Applications by Bernard Sklar (1988). For systems with rapid acquisition requirements, such as the digital cellular system, the data packet has a synchronization codeword. A corresponding codeword in the receiver is correlated to the data packet until it is matched with the synchronization codeword. A data packet having multiple sampling points per symbol is synchronized to the receiver using the correlation process to determine the sampling point for all the symbols in the data packet. The sampling point of a symbol corresponds to the value of the detected sample when it is closest to one of the eight points on the phase constellation. Symbols recovered at the sampling point have the best chance of being detected correctly in the presence of channel-induced distortion to minimize the receiver's bit error rate (BER) performance.
Unfortunately, variations in amplitude over the duration of the data packet due to the channel-induced distortion can cause the sampling point for symbol recovery to vary. This situation is particularly apparent in data packets having a long time duration such as in the digital cellular system. Correlation of the receiver to the codeword in the data packet reflects channel conditions only for the instant in time at which correlation occurred. The sampling point determined from correlation codeword in one portion of a data packet may not be optimal for symbols in another portion of the data packet. Additionally, some or all of the symbols in the synchronization codeword may be distorted resulting in such poor correlation that the sampling point for symbol recovery would be based upon an estimated sampling point. For an operator of a receiver using this type of radio system in a time-varying channel, the estimated sampling point may cause distorted audio reception or loss of transmitted data or control information.
For many situations, of which a cellular radiotelephone is merely an example, the prior art has not produced an apparatus or method of information recovery to meet the difficult requirement of providing a substantially valid sampling point over the duration of a time-varying signal.